1. Field of the Invention
The invention relates to an electric power steering system.
2. Description of the Related Art
An electric power steering system has been developed, in which a vehicle speed is detected by a vehicle speed sensor, a steering torque applied to a steering wheel is detected by a torque sensor, a steering assisting motor is driven by being energized with a driving electric current whose value is determined according to the steering torque and the vehicle speed detected, and the force required to steer a vehicle is assisted by the torque of the motor to provide a driver with a comfortable steering feeling.
There is a case where an unexpected load is applied to a constituent member of the electric power steering system due to an erroneous operation performed by the driver while the vehicle is running. This is, for example, a situation where a steered wheel of the vehicle hits a curbstone at the edge of a roadway. When a steered wheel of the vehicle hits the curbstone, an external force is applied to the steered wheel and there is a possibility that the steered wheel is forcibly steered in one direction. When the steered wheel is forcibly steered, the rack shaft, which is a constituent element of the steering system, is forcibly moved in the axial direction. Depending on how the steered wheel hits the curbstone, there is a case where a base portion of a tie rod at an end of the rack shaft hits a mechanical stopper at a high speed, which is provided to prevent the rack shaft from moving beyond the allowable moving range in the axial direction.
When the base portion of the tie rod at an end of the rack shaft hits the mechanical stopper, an inertial force of the rotary shaft of the steering assisting motor is applied to a constituent part of the torque transmitting system as an impulsive force. Thus, there is a possibility that a large impulsive force is applied to constituent parts of the torque transmitting system, such as a speed reducing portion that reduces the speed of rotation of the motor and a steering shaft, which can cause a failure.
Japanese Patent Application Publication No. 6-8839 (JP-A-6-8839) describes a system, in which when the steering speed becomes equal to or higher than a predetermined setting value of the steering speed, a clutch is disengaged and the inertial force of an electric motor is therefore not transmitted to a steering mechanism, so that the inertial force of the electric motor is not added to the steering force occurring in the steering system when a curbstone is hit.
However, the system described in JP-A-6-8839 requires a clutch mechanism and therefore, the structure is complicated and the manufacturing costs are high.
Japanese Patent Application Publication No. 2008-24277 (JP-A-2008-24277) describes a system, in which when the rate of change in the motor torque detected by a motor torque detector is equal to or higher than a threshold value that is used to determine whether the steering limit position is reached, it is determined that a duty factor limiting condition is satisfied, and the duty factor of the pulse width modulation signal is fixed at a predetermined value that is used to restrict the torque that is transmitted to a torque transmitting member between a steered wheel and a steering shaft of the steering mechanism. According to this system, the steering assisting torque produced by the electric motor is limited before an excessive torque is transmitted to a torque transmitting member, such as an intermediate shaft, that is interposed between the steering shaft and a steering gear. Thus, it is possible to suppress the impulsive force that is transmitted to the torque transmitting member when the steering limit position is reached, such as when a tire hits a curbstone, without providing an additional steering angle sensor, an additional torque limiter, etc.
In addition, JP-A-2008-24277 describes a system, in which when the rate of change in the motor torque detected by a motor torque detector is equal to or higher than a threshold value that is used to determine whether the steering limit position is reached, the duty factor is fixed at a predetermined value to suppress the impulsive force that is transmitted to a torque transmitting member when the steering limit position is reached, such as when a steered wheel hits a curbstone. When the steering limit position is reached after the steered wheel hits the curbstone, however, it is impossible to surely suppress the impulsive force that is transmitted to the torque transmitting member because even when the electric current is limited as described above, the motor is rotating at a speed equal to or higher than a no-load rotation speed.